1. Field of the Invention
The present invention relates to a semiconductor device and its manufacturing method and, in particular, to a semiconductor device having a metallic fuse member that can be cut by laser light and the manufacturing method thereof.
2. Description of the Background Art
Conventionally, fuses for use in redundant circuit replacement have been formed by a wiring layer comprising a polysilicon or a silicide underlying a metal wiring layer.
The technology of multi-layer wiring structure has advanced as well as the large scale integration of memories. Specifically, 1Mega-bit Dynamic Random Access Memory (hereinafter referred to as "1MDRAM") comprises three polysilicon wiring layers and a single Al wiring layer, 4MDRAM and 16MDRAM comprise four polysilicon wiring layers and two aluminum wiring layers, and the generations from 64MDRAM may have five polysilicon wiring layers and three aluminum wiring layers.
In conventional wiring layers comprising polysilicon or silicide that have been used as a fuse for a redundant circuit replacement, as the number of wiring layers is increased, the entire thickness of insulating films overlying the respective wiring layers is increased. This makes it difficult to carry out a stable blow of laser light, causing the difficulty in serving as a fuse.
More specifically, due to the presence of the aforesaid thick insulating film, laser light blow requires a large amount of laser light energy, which then damages an insulating film underlying a fuse, or a silicon substrate thereunder, and therefore causes an electrical leakage between the fuse and the substrate. This results in the malfunction of a redundant circuit.
To solve the above problem, there is a well-known technique in which a thick portion of insulating film is provided directly under a fuse. With this technique, the volume of this thick portion is relatively greater than that of other portions. Thus, even if a relatively large laser light is irradiated, the above stated problem does not occur.
In this technique, however, another processing step for forming a thick portion of insulating film is required and the irradiation energy of laser light cannot be reduced.
Japanese Patent Laid Open Gazette 8-213465 discloses a technique for obtaining semiconductor devices having a fuse member that can be cut by laser light of a relatively small energy, without additional processing steps.
FIGS. 6A and 6B are a diagram for explaining a semiconductor device described in the aforesaid publication; FIG. 6A is a plan view of a detailed part of the semiconductor device; and FIG. 6B is a cross-sectional view along line B--B of FIG. 6A.
Referring to FIGS. 6A and 6B, a fuse member 71 comprises a metal that can be cut by laser light, e.g., aluminum. Contact holes 72a, 72b make an electrical connection between the ends of the fuse member 71 and the underlying wiring layers 73a, 73b, respectively. An insulating film 74 is formed so as to envelop the fuse member 71 and the wiring layers 73a, 73b. Laser light 75 with an irradiation spot diameter D1 is irradiated to the fuse member 71. Here, the length L1 of the fuse member 71 is set so as to satisfy the condition L1.ltoreq.D1.
In conventional semiconductor devices with the above construction, it is able to cut the fuse member 71 by the laser light 75 of a relatively small energy, without adding a special processing step.
However, these semiconductor devices have the problem that when a metal wiring layer is used as a fuse member, part of the fuse member, i.e., part of the metal, remains even after laser blow, and in a moisture proof test the remaining metal is corroded to affect the adjacent fuse, causing the malfunction of a redundant circuit.
To avoid such a corrosion, it has been proposed to cover with a silicon nitride film or the like after blowing a fuse member. However, this method is accompanied by an increase in the number of processing steps.